Molecular dynamics study of the effects of chain properties on the order formation dynamics of self-assembled...

The order formation dynamics of self-assembled monolayers (SAM) of long-chain molecules were studied using coarse-grained molecular dynamics simulations. The primary kinetic processes of surface order formation from solution are adsorption to the surface and surface diffusion. For long-chain molecules, the degrees of freedom of the chain structure and motion add various complexities to the order formation dynamics. Specifically, the strength of the chain interaction, the chain flexibility and the chain length play a significant role, and this work focused on the effects of these chain properties on the order formation dynamics. The adsorption dynamics of SAM molecules can be explained by the same theoretical framework as the polymer brush. On the other hand, the evolution of highly ordered structure is specific to SAM systems. Simulation results revealed that the development of oriented domains can be grouped into three types, isolated island growth, packing growth, and growth suppression, which depend on temperature and chain flexibility. In packing growth, oriented domains are formed gradually due to the decrease in free volume as the surface density becomes high, while the tilt of the adsorbed chain molecules does not become upright gradually as a whole. Rather, inside the oriented domains, the adsorbed chains adopt "standing" states with tilt angles almost equal to the final values, which contributes to the gradual increase in the total tilt order. The effect of chain length was also studied. In the case of semirigid chain molecules, longer-chain systems showed slightly slower growth in adsorption but faster growth in oriented domains. These simulation results reveal how chain properties influence the dynamics of oriented structure formation on surfaces.